UIO: fix uio_pdrv_genirq with device tree but no interrupt

If device is initialized from device tree, but has no interrupt
assigned, uio will still try to request and interrupt old way,
fails, and fails registration.

This is wrong; don't try initializing irq using platform data if
device tree is available.

Simplified code based on suggestion by Grant Likely.

Fixed memory leak in "irq can not be registered" error path.

Signed-off-by: Pavel Machek <pavel@denx.de>
Reported-by: Detlev Zundel <dzu@denx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>